It turns out Frumin also has a blog. It's irregular, but interesting, with posts about things he's working on, like MTA Bus Time, and also some calculations of how much vehicle capacity the NYC subway system saves. Here's his conclusion:

Just to get warmed up, chew on this -- from 8:00AM to 8:59 AM on an average Fall day in 2007 the NYC Subway carried 388,802 passengers into the CBD [Central Business District, ie, Manhattan below 60th Street] on 370 trains over 22 tracks. In other words, a train carrying 1,050 people crossed into the CBD every 6 seconds. Breathtaking if you ask me.

Over this same period, the average number of passengers in a vehicle crossing any of the East River crossings was 1.20. This means that, lacking the subway, we would need to move 324,000 additional vehicles into the CBD (never mind where they would all park).

And that map at the top? It's a screenshot from Frumin's post showing the change in ridership at MTA stations going back to . . . 1905. Here he's mapped the stations around Williamsburgh, Brooklyn, but he also shows the areas around Yankee Stadium and Wall Street.